Observations of living cells or tissues through fluorescence microscopy provide insights into dynamic behavior and cellular physiology that cannot be obtained from observations of fixed specimens. Such information may reveal the pathological state of a tissue to the physician or information on cytoskeletal dynamics to the research scientist. Whole genome sequencing combined with the use of fluorescent protein reporters has opened up the possibility of visualizing the distribution and dynamics of any protein in living tissue. Furthermore, the use of specific fluorescent indicators has allowed the physiological state of a cell to be visualized in vivo. However, problems of overlapping spectra, low signal and light scatter pose serious limitations on what can be achieved in practice with fluorescence microscopy. These problems will be addressed by the development of a multi-photon imaging system that makes maximum use of all the information present in the fluorescence signal. A novel photon-counting detector will be developed that simultaneously measures the wavelength and lifetime of individual fluorescent photons. Algorithms will be developed to analyze and extract physically meaningful characteristics of individual fluorescent components from noise-limited signals. The system will be applied to cell biological studies using fluorescent reporters and to the identification of potentially diagnostic spectral/lifetime signatures of cells and tissue culture models of cancer.